1. Field of the Invention
The present invention is an invention developed from U.S. patent application Ser. No. 11/434,107, especially relates to an apparatus with the capability of adjusting and controlling the magnetic state of a probe. It uses a negative feedback electronic unit to compensate the change of magnetic moment in the probe by applying a current to the feedback coil inductively coupled with the probe. This coil can also serve as a bias coil to keep the probe at an arbitrary magnetic state. Therefore, this apparatus can control the magnetic interaction between the magnetic probe and the sample.
2. Description of the Prior Art
A scanning probe microscope (SPM) which its probe measures physical quantities and records them a function of position on the sample surface can observe phenomena in the micro world. Detecting magnetic signals in an SPM requires the magnetic interaction between the tip and the sample. The local magnetic field or flux of a sample experienced by the tip can be detected by a specific sensor. For example, the magnetic tip on the cantilever of a magnetic force microscope (MFM) can sense the magnetic state of a sample through either the frequency shift of the vibration or the deflection of the cantilever. In a conventional MFM, a mapping of the magnetic state of sample surface is achieved by the scanning of the tip over the surface. A scanning superconducting quantum interference device (SQUID) microscope (SSM) disclosed in Appl. Phys. Lett. 66, 1138 (1995) by J. R. Kirtley et al. detects a magnetic flux through the pick-up coil coupled to the SQUID that is highly magnetic-sensitive. The employment of a magnetic flux guide in an SSM [Ref. J. R. Kirtley, Physica C 368, 55-65 (2002).] gives rise to the flexibility of an environment placing sample and a SQUID chip. The flux guide, usually made of permalloy, couples the magnetic flux of the sample with the SQUID chip. Through the same methodology of SPM, SSM can measure the delicate magnetic distribution of the sample.
In measuring the magnetic distribution, the magnetic force is exerted on the tip of MFM through the stray field of the sample. Since the magnetization of a material depends on the external magnetic field, the magnetic interaction between the tip and sample may alter either the magnetic state of the sample or the tip during the scan. The current solution to prevent the alteration of the magnetization of the tip is to employ a hard ferromagnetic tip that has a large remanence near the saturation induction. However, it does not solve the problem of the sample; for example, the relative movement between the tip and the sample would drive the domain walls of a soft magnetic film to change when the probe is close enough to the sample. The alteration of sample magnetic distribution can be alleviated by increasing the tip-sample separation, though a degeneration of the spatial resolution is disadvantageous. This dilemma remains to be a tough issue in the field of magnetic force microscopy. The high-permeability tip in a flux-guide SSM can be magnetized by the magnetic field of the sample when the tip approaches the sample. As a picture of magnetic field distribution shows, the tip absorbs the magnetic flux [Ref. T. Kondo and H. Itozaki, Physica C 392-396, 1401-1405 (2003).]. It generates an attractive force between the tip and the local magnetic moment of the sample. In the case of a weak-pined moment, the moving tip can drag the magnetic moment. Again, the magnetic probe disturbs the magnetic distribution of the sample.
U.S. Pat. No. 5,331,491 discloses a magnetic recording head with a soft magnetic needle and an exciting coil wound around the needle. The magnetic needle is a medium that transfers the magnetic signal generated by the exciting coil and conducts the electromagnetic wave emitted by the high-frequency oscillator. To write a signal, it applies an electric current to the exciting coil to magnetize the needle and then records the signal on a magnetic recording medium. The sensing process detects the difference between the wave generated by the oscillator and the wave reflected from the magnetic recording medium, which is influenced by the local magnetic field of the recording medium through the variation of the magnetic permeability of the soft magnetic needle.
U.S. Pat. No. 4,677,512 discloses a magnetic reproducing apparatus including a magnetic guide and two coils, one for writing magnetic signals and the other for the inductor in the tuning circuit for detecting magnetic signals. The magnetic guide induces the magnetization of signals to be recorded on the sample via applying a current in the writing coil wound on the guide. The variation of the magnetic field changes the permeability of the guide where the second coil is disposed in its proximity and thus the inductance of the coil is altered. To detect the magnetic signal in the sample, the frequency shift in the RLC tuning circuit is measured. These two coils function as the component inducing magnetization in the magnetic guide and an inductor in the detecting circuit respectively.
Among the prior arts of magnetic apparatuses, they seldom deal with the problem caused by the magnetic interaction between the probe and the sample. This would be a serious issue in measuring a delicate magnetic signal.